In so-called color Doppler imaging which generates a spatial distribution associated with the moving velocity of a moving object such as a blood flow, it is necessary to repeat ultrasonic transmission/reception a plurality of numbers of times in the same ultrasonic raster (to be simply referred to as raster hereinafter) direction. For this reason, when trying to secure a certain number of frames per second (frame rate), it is not possible to obtain a sufficiently large number of rasters. There is a tradeoff relationship between frame rates (time resolutions) and the numbers of rasters (spatial resolutions).
Raster interpolation is a method of suppressing a degradation in time resolution while maintaining a certain spatial resolution. The mainstream technique of raster interpolation is to directly interpolate the final average velocity of a blood flow or the like. If, however, for example, the pieces of velocity information of adjacent rasters are +α and −α, simply averaging them will result in zero velocity. This may generate an unnatural black part on the interpolated raster. As a method of improving this situation, there is available a raster interpolation method using complex data obtained by quadrature detection, as exemplified by FIGS. 6 and 7. Raster interpolation processing (complex interpolation) on this complex data takes initial amplitudes into consideration. When, therefore, converting the data into an average velocity, it is possible to improve the estimation accuracy of interpolated velocity components.
In complex interpolation, however, a degradation in interpolation accuracy with a velocity change, in particular, appears as an artifact in the form of a vertical streak on the image, as shown in FIG. 8.